Process for coating sheet substrates with thermoplastic polymer

ABSTRACT

An improved process for coating sheet substrates with thermoplastic polymer is disclosed. In the process the sheet substrate forms one die lip of the extrusion coating die. The sheet substrate is passed from face-to-face contact with the metal block of the extrusion coating die into a spaced apart relationship with one die lip of the extrusion coating die, the sheet substrate forming the second die lip. Molten thermoplastic polymer is extruded into the space between the one die lip and the sheet substrate, thereby coating the sheet substrate with polymer. The preferred polymer is a poly-α-olefin, especially polyethylene or a copolymer of ethylene and butene-1. The preferred sheet substrate has orifices in the surfaces thereof. The process may be used to coat sheet substrates with a thermoplastic polymer, e.g. in the manufacture of form boards for the construction industry.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for coating sheet substrateswith thermoplastic polymer and in particular to an improved process forcoating sheet substrates with a relatively thick coating ofthermoplastic polymer in which the sheet substrate forms one die lip ofthe extrusion coating die employed in the coating process.

2. DESCRIPTION OF THE PRIOR ART

Techniques for the coating or lamination of a sheet substrate withthermoplastic polymer to form coated panels are known. For example thesubstrate and the thermoplastic polymer in the form of film or sheet maybe brought into contact and laminated by the application of heat andpressure. Alternatively, the sheet substrate may be coated withthermoplastic polymer in a heated, and especially in a molten form.Adhesion of the thermoplastic polymer to the substrate is usuallyenhanced by applying pressure to the coated substrate before thethermoplastic polymer cools. Adhesives may be applied to the substratebefore the substrate is coated with the thermoplastic polymer.Alternatively, adhesion-promoting compounds may be applied to thesubstrate or incorporated into the thermoplastic polymer.

Thermoplastic polymer-coated sheet substrate in the form of coatedpanels may be used in a variety of end uses. For example,polyolefin-coated wooden panels may be used as concrete form boards, asis disclosed by D. H. Dawes and J. P. van den Steen in Canadian Pat. No.931,486 which issued Aug. 7, 1973. Coated panels may also be used in theconconstruction industry in interior and exterior applications.Techniques for coating wooden structures so as to obtain products ofimproved properties are disclosed in U.S. Pat. No. 3,544,669 of F.Schock which issued Dec. 1, 1970.

Coated panels having a coating of thermoplastic material of a thicknessof at least 0.25 mm on a substantially inflexible substrate aredisclosed in U.S. Pat. No. 4,004,774 of A. J. Houston which issued Jan.25, 1977. In the coated panels disclosed by A. J. Houston, the coatingof thermoplastic material is mechanically attached to the substrate. Thesubstrate has a plurality of orifices in the surface thereof that is tobe coated, the orifices being of a shape such that at least in part, thecross-sectional area of the orifice increases as the distance from thesurface of the substrate increases. The coating of thermoplasticmaterial is applied so that the thermoplastic material extends into theorifices at least as far as the portion of the orifices of increasingcross-sectional area.

An improved process for the coating of sheet substrates withthermoplastic polymer so as to form coated panels, especially the coatedpanels disclosed by A. J. Houston, has now been found.

SUMMARY OF THE INVENTION

Accordingly the present invention provides in a process for coating asubstantially inflexible sheet substrate with a coating of thermoplasticpolymer in which molten thermoplastic polymer is extruded between firstand second die lips of an extrusion coating die and onto a face of saidsheet substrate, said extrusion die being formed, in part of a metalblock having means to feed molten polymer therethrough to said die lips,the improvement comprising feeding the sheet substrate in face-to-facecontact with said metal block over substantially the entire width ofsaid sheet substrate, passing said sheet substrate from contact with themetal block into spaced apart relationship with the first die lip, saidsheet substrate thereby forming the second die lip of the extrusioncoating die, extruding molten thermoplastic polymer into the spacebetween the first die lip and the sheet substrate and thereby coatingthe sheet substrate with thermoplastic polymer.

In a preferred embodiment of the process of the present invention, thesheet substrate has a plurality of orifices in the surface thereof thatis to be coated, the orifices being of a shape such that, at least inpart, the cross-sectional area of the orifices increases by at least tenpercent as the distance from the surface of the substrate increases.

In a further embodiment the thermoplastic polymer is a polyolefinespecially a homopolymer of ethylene or a copolymer of ethylene andbutene-1.

In another embodiment the coating on the substrate is, prior tosolidification of the molten thermoplastic polymer, contacted with atleast one roll, the roll preferably having a smooth surface.

DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of the cross-section of oneembodiment of the present invention.

FIG. 2 is a schematic representation of the cross-section of anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As stated above the present invention relates to the coating of sheetwith a thermoplastic polymer. The invention will be describedhereinafter with particular reference to FIGS. 1 and 2.

With reference to the embodiment shown in FIG. 1 the apparatus used inthe embodiment comprises an extrusion coating die, indicated generallyby 1, a sheet substrate 2, and adjustable platform 3 and supports 4. Theextrusion coating die 1 has a metal block 5 with a planar bottom surface6 in face-to-face contact with the upper surface 7, i.e. the surfacethat is to be coated with thermoplastic polymer, of sheet substrate 2.Planar bottom surface 6 of metal block 5 has a number of recesses 8therein. In recesses 8 are located rotatable ball 9 and spring 10.Rotatable ball 9 is held in contact with surface 7 of sheet substrate 2by spring 10 and facilitates the movement of sheet substrate 2 withrespect to planar bottom surface 6 of metal block 5.

Metal block 5 has a channel 11 in fluid communication with a source (notshown) of molten thermoplastic polymer. Channel 11 is, in turn, in fluidflow communication with die gap 12, die gap 12 being the space formedbetween the surface 7 of sheet substrate 2 and upper die lip 13. Upperdie lip 13 is attached to metal block 5 and is located in the side ofmetal block 5 that is away from that adjacent to recesses 8 in theplanar bottom surface 6 of metal block 5. Die gap 12 is adjustable by,for example, adjustment of the upper die lip 13, such adjustment beingaccomplished by die lip adjustment means 14.

Choker bar 15 extends through metal block 5 and partially into channel11. The extent to which choker bar 15 extends into channel 11 isadjustable using choker bar adjustment 16. Sheet substrate 2 issupported by adjustable platform 3 and supports 4. The position ofadjustable platform 3 with respect to extrusion coating die 1 may beadjusted using worm screw 17.

In the embodiment shown two rolls 18 and 19 are shown to be locatedadjacent to upper die lip 13. The surfaces of rolls 18 and 19 are incontact with coating 20 on sheet substrate 2. Air cooling means 21 islocated between roll 18 and roll 19 and spaced apart from coating 20.The air cooling means may also be located before roll 18 or after roll19, or any combination thereof.

In the embodiment shown the sheet substrate 2 has a plurality oforifices 22 located therein. Orifices 22 are of smaller cross-sectionalarea near the surface 7 of sheet substrate 2 than at a position awayfrom the surface.

In the embodiment shown in FIG. 2 the apparatus comprises an extrusioncoating die, indicated generally by 31, a sheet substrate 2, supportrolls 32 and 33 and drive roll 34. The extrusion coating die 31 has ametal block 35 with a planar bottom surface that consists of twosections viz. rear extended die lip 36 and front extended die lip 37.Rear extended die lip 36 is in face-to-face contact with upper surface7, i.e. the surface that is to be coated with thermoplastic polymer, ofsheet substrate 2. Front extended die lip 37 is in spaced apartrelationship, but essentially parallel to, upper surface 7 of sheetsubstrate 2. The space between front extended die lip 37 and sheetsubstrate 2 forms die gap 38.

Metal block 35 has a channel 39 in fluid flow communication with die gap38, channel 39 being connected to a source (not shown) of moltenpolymer.

Support rolls 32 and 33 are located under sheet substrate 2 oppositeextrusion coating die 31. Drive roll 34 is located beyond the end offront extended die lip 37 and on the opposite side of sheet substrate 2thereto. Surface roll 40 is located opposite drive roll 34 i.e. on thesame side of sheet substrate 2 as front extended die lip 37. Surfaceroll 40 is in contact with coating 41.

Air ring 42 is located above coating 41 and adjacent to surface roll 40.Air ring 42 is on the opposite side of surface roll 40 to front extendeddie lip 37.

In operation of the apparatus of FIG. 1, the position of adjustableplatform 3 is adjusted so that sheet substrate 2 will be in slidingengagement with both planar bottom surface 6 of metal block 5 andadjustable platform 3. Sheet substrate 2 is passed, using driving meansnot shown, in sliding engagement with planar bottom surface 6 andadjustable platform 3, such passage being facilitated by rotatable balls9. Sheet substrate 2 then passes into spaced apart relationship withupper die lip 13, thereby forming die gap 12. Molten thermoplasticpolymer is extruded through channel 11 into die gap 12. The rate of flowof thermoplastic polymer may be adjusted in part using choker bar 15. Ifthe sheet substrate 2 has orifices 22, as is shown in the embodimentillustrated in FIG. 1, the pressure of the thermoplastic polymer as theresult of extrusion of the polymer through channel 11 forces polymerinto orifices 22. The thickness of the coating may be adjusted by inparticular adjusting the position of upper die lip 13. The coating 20 ofmolten thermoplastic polymer thus formed on sheet substrate 2 thencools, or is cooled by external means, thereby forming a solidifiedcoating of thermoplastic polymer on the sheet substrate.

The sheet substrate 2 may be used in relatively long lengths with thecoated substrate being cut into desired length or the sheet substrate 2may be pre-cut into the desired lengths and fed to the extrusion coatingdie in an essentially abutting series of sheet substrate sections.

In a preferred embodiment the coating 20 is contacted with at least onedriven roll, rolls 18 and 19 of FIG. 1, after coating 20 becomes out ofcontact with upper die lip 13. The rolls preferably have smooth surfacesand are adapted to provide a smooth surface on coating 20. Coating 20may also be cooled using air cooling means 21. In a particularlypreferred embodiment as the coated substrate passes from the extrusioncoating die, the still molten coating is contacted with, in series, roll18, cooling air from air cooling means 21 and roll 19.

In another embodiment an auxiliary heater is located in upper die lip13. The auxiliary heater may be used to reduce or eliminate "cold spots"in the extruded coating and to reduce or prevent sticking of the coating20 to upper die lip 13.

The apparatus of FIG. 2 is operated in a manner similar to that ofFIG. 1. Sheet substrate 2 is passed, using drive means not shown and/ordrive roll 34, in sliding engagement with rear extended lip 36 andsupported by rolls 32 and 33. Sheet substrate 2 is in spaced apartrelationship with front extended lip 37, thereby forming die gap 38.Molten thermoplastic polymer is extruded through channel 39 into die gap38. The rate of flow of thermoplastic polymer may be controlled withinchannel 39 by means not shown. If sheet substrate 2 has orifices 22, asis shown in FIG. 2, the pressure of the thermoplastic polymer as theresult of extrusion of the polymer through channel 39 forces polymerinto orifices 22. The thickness of coating 41 may be adjusted by inparticular adjusting the position of front extended lip 37.

Roll 40 is preferably a smooth roll so as to aid in smoothing thesurface of coating 41 although roll 40 may be adapted to emboss thesurface of coating 41. In the embodiment shown in FIG. 2 air ring 42 isadapted to cool both coating 41 and roll 40.

The sheet substrate 2 may be any substantially inflexible sheetsubstrate that will not melt or be significantly degraded at thetemperatures used in the extrusion of the thermoplastic polymer. Thesubstrate preferably has a plurality of orifices in the surface thereofthat is to be coated. The orifices preferably are of a shape such that,at least in part, the cross-sectional area of the orifices increases byat least ten percent as the distance from the surface of the substrateincreases. The orifices are preferably of a cross-sectional area ofgreater than 0.035 sq. cms. The depth of the orifices may be equal to orless than the thickness of the sheet substrate. The preferred substrateis wood but substrates of metal or of a polymer, especiallythermoplastic polymer, that does not melt or significantly degrade atthe temperatures used in the process may be used.

The coating applied to the substrate is preferably of a thickness of atleast 0.25 mm and especially 0.5-3.0 mm.

The thermoplastic polymer used for coating 20 must be capable of beingextruded and must be solid under the conditions of the proposed end useof the coated substrate. Preferred thermoplastic polymers arepolyamides, for example polyhexamethylene adipamide, polystyrene,polyvinyl chloride and poly-α-olefins, for example polypropylene andespecially homopolymers of ethylene and copolymers of ethylene andα-olefins, for example, butene-1 or vinyl acetate.

The thermoplastic polymer may contain fillers, pigments, stabilizers,foaming agents and/or reinforcing agents. For homopolymers of ethyleneand copolymers of ethylene and butene-1 the melt index is preferably inthe range 0.2-30.0 and especially in the range 0.3-7.5, melt index beingmeasured by the method of ASTM D-1238 (Condition E). Preferredhomopolymers of ethylene and copolymers of ethylene and butene-1 arethose of densities of at least 0.935 g/cm³ and especially at least 0.945g/cm³ and in particular such polymers having a broad molecular weightdistribution.

Suitable polyethylenes may be obtained by any of the well knownpolymerization processes; using a coordination catalyst is aparticularly useful route which may be readily adapted to providehomopolymers of ethylene as well as copolymers of ethylene and otherα-olefins, for example butene-1.

Processes for the extrusion coating of substrates, especially woodensubstrates having orifices, are susceptible to a number of potentialprocess problems. The potential problems include tendencies for "sinkmarks" or dimples to appear in the surface of the coating at thelocation of the orifices in the substrate, for incomplete filling of theorifices and for the wooden substrate to warp and thus become nonplanar.As is illustrated hereinafter the present invention is capable ofproducing coated substrates of higher quality than those from aso-called "conventional" coating process in which the die is positionedabove the substrate and the polymer is extruded down and onto thesubstrate. Conventional processes often include a smoothing roll similarto roll 40 described hereinabove, excess polymer being allowed toaccumulate adjacent to the smoothing roll to aid in the attaining of auniform coating.

In the process of the present invention the orifices may readily befilled with the thermoplastic polymer. Moreover the front extended dielip of the die significantly reduces any tendencies for the substrate towarp and for sink marks to form if the substrate has orifices.

The length of the extension of the die lips is a factor in the operationof processes of the present invention. Preferably the length of thefront extended die lip is at least 10 cm and in particular at least 15cm.

In addition to the use of the extended die lips, the use of air ringsand especially the rate of cooling of the coating achieved thereby maybe a factor in the formation of sink marks in the coating.

If the sheet substrate is wood, it is preferable to dry the surfacethereof before coating the substrate according to the process of thepresent invention.

While the process of the present invention has been describedhereinabove with particular reference to the coating of substrateshaving orifices, the process may be used with substrates having noorifices or with substrates having slits in the surface thereof,especially alternating elongated slits at angles of about 45° and about135° to the surface.

The process of the present invention is particularly useful for themanufacture of the coated panels described in the aforementioned U.S.Pat. No. 4,004,774 of A. J. Houston. In particular the process of thepresent invention may be used in the manufacture of polyethylene-coatedwooden form boards for use in the construction industry.

The present invention is illustrated by the following examples:

EXAMPLE I

Plywood sheets having a width of 15 cm and a thickness of 1.25 cm andhaving orifices spaced apart at approximately 2.5 cm centres, theorifices being of a diameter of 0.62 cm extending therethrough andreamed to a diameter of 1.2 cm on the side of the plywood that was notto be coated, were dried in an oven at 121° C. for 30 minutes. The driedplywood sheets were then coated using the apparatus illustrated in FIG.2 and described hereinabove.

The thickness of the coating was approximately 1.8 mm. The coating couldbe removed from the coated plywood sheets only if the polymer wassheared at the orifices.

In order to test the coated plywood, samples of the coated plywood wereimmersed in water for a period of time. The panels were then examinedfor cracks extending from an orifice to an adjacent orifice.

Further details and the results obtained are given in Table I.

                  TABLE I                                                         ______________________________________                                        Run           1      2      3    4    5    6                                  Polymer*      A      B      C    D    E    F                                  ______________________________________                                        Polymer Temperature                                                                         287    271    237  237  237  237                                (°C.)                                                                  Melt Pressure                                                                 (Kg/cm.sup.2) in die                                                                        4390   4390   4460 4480 3950 3950                               Speed of Plywood                                                                            1.5    1.5    1.2  1.2  1.8  1.8                                (m/min)                                                                       Orifices Filled With                                                          Polymer (%)   100    100    100  100  100  100                                Warpage of Coated                                                             Plywood       none   none   none none none none                               Sink Marks in                                                                 Coating       none   none   none none none none                               Testing of Samples                                                            (a) Temperature of                                                            Water (°C.)                                                                          80     80     80   80   80   80                                 Test Time (hours)                                                                           24     24     24   24   24   24                                 Appearance of yes    yes    no   no   yes  yes                                Cracks                                                                        (b) Temperature of                                                            Water (°C.)                                                                          20     20     20   20   20   20                                 Test Time (hours)                                                                           624    624    624  624  624  624                                Appearance of                                                                 Cracks        no     no     no   no   no   no                                 ______________________________________                                         *Polymer A  ethylene homopolymer having a density of 0.960, a melt index      of 0.75 and a broad molecular weight distribution                             Polymer B  ethylene homopolymer having a density of 0.950, a melt index o     4.0 and a medium molecular weight distribution                                 Polymer C  ethylene/butene1 copolymer having a density of 0.943, a melt      index of 1.25 and a broad molecular weight distribution                       Polymer D  ethylene/butene1 copolymer having a density of 0.946, a melt       index of 0.35 and a broad molecular weight distribution                       Polymer E  ethylene homopolymer having a density of 0.956, a melt index o     1.0 and narrow molecular weight distribution                                  Polymer F  ethylene/butene1 copolymer having a density of 0.947, a melt       index of 2.49 and a medium molecular weight distribution.                

EXAMPLE II

As a comparison the plywood of Example I was coated using a conventionalprocess. A flat film die was located above the plywood at a height of0.6 cm. Molten polymer was extruded onto the plywood and was allowed toaccumulate adjacent to a smoothing roll located approximately 5 cm fromthe die.

Further details and the results obtained were as follows:

    ______________________________________                                        Run            7        8        9      10                                    Polymer**      G        H        I      J                                     ______________________________________                                        Polymer Temperature                                                                          240      240      225    240                                   (°C.)                                                                  Speed of Plywood                                                                             1.5      1.5      1.5    1.5                                   (m/min)                                                                       Coating Thickness                                                                            1.2      1.8      1.8    2.1                                   (mm)                                                                          Orifices filled with                                                                         50       NM       30     0                                     Polymer (%)                                                                   Warpage of Coated                                                                            yes      yes      yes    yes                                   Plywood                                                                       Sink Marks in  yes      yes      yes    yes                                   ______________________________________                                         Coating-                                                                      **Polymer G  ethylene/butene1 copolymer having a density of 0.946, a melt     index of 5 and a narrow molecular weight                                      Polymer H  ethylene homopolymerr having a density of 0.960, a melt index      of 0.4 and a broad molecular weight                                           **Polymer I  ethylene homopolymer having a density of 0.960, a melt index     of 5 and a narrow molecular weight                                            Polymer J  ethylene homopolymer having a density of 0.955, a melt index o     2.5 and a medium molecular weight                                             NM  not measured but less than 50%.                                      

EXAMPLE III

Plywood sheets of the type described in Example I were coated withextrusion grade polyvinyl chloride having a specific gravity of 1.37 anda Shore A hardness of 85 and which had been obtained from CarlewChemicals Limited of Montreal, Que., under the trade designation Y. 371.Nat. The sheets were coated at a speed of 0.5 m/min using the proceduredescribed in Example I. The melt temperature of the polyvinyl chlorideon extrusion was 216° C.

The polyvinyl chloride coating extended through the orifices in thesheet to the part thereof that had been reamed. The coating, which had athickness of approximately 0.8 cm, could only be removed from the coatedplywood sheets if the polymer was sheared at the orifices.

We claim: .[.1. In a process for coating a substantially inflexiblesheet substrate with a coating of thermoplastic polymer in which moltenthermoplastic polymer is extruded between first and second die lips ofan extrusion coating die and onto a face of said sheet substrate, saidextrusion die being formed, in part, of a metal block having means tofeed molten polymer therethrough to said die lips, the improvementcomprising feeding the sheet substrate in face-to-face contact with saidmetal block over substantially the entire width of said sheet substrate,passing said sheet substrate from contact with the metal block intospaced apart relationship with the first die lip, said sheet substratethereby forming the second die lip of the extrusion coating die, thefirst die lip extending substantially parallel to and in spaced apartrelationship from the sheet substrate for a distance of at least 10 cm,said sheet substrate having a plurality of openings selected from thegroup consisting of orifices and slits in the surface thereof, extrudingmolten thermoplastic polymer into the space between the first die lipand the sheet substrate and thereby coating the sheet substrate withthermoplastic polymer..]. .[.2. The process of claim 1 in which thethermoplastic polymer is selected from the group consisting ofpolyamides, polystyrene, polyvinyl chloride and poly-α-olefins..]. .[.3.The process of claim 2 in which the thermoplastic polymer is polyvinylchloride..]. .[.4. The process of claim 2 in which the thermoplasticpolymer is a poly-α-olefin..]. .[.5. The process of claim 4 in which thepoly-α-olefin is selected from the group consisting of homopolymers ofethylene and copolymers of ethylene and butene-1..]. .[.6. The processof claim 5 in which the sheet substrate is wood..]. .[.7. The process ofclaim 5 in which that part of the metal block which contacts the sheetsubstrate in face-to-face contact is a rear extended die lip..].
 8. Theprocess of claim .[.5.]. .Iadd.15 .Iaddend.in which the openings areorifices, said orifices being of a shape such that, at least in part,the cross-sectional area of the orifices increases as the distance fromthe surface of the substrate increases. .[.9. The process of claim 8 inwhich the substrate is wood..].
 10. The process of claim .[.9.]..Iadd.15 .Iaddend.in which the first die lip extends substantiallyparallel to and in spaced apart relationship to the sheet substrate fora distance of at least 15 cm.
 11. The process of claim .[.9.]. .Iadd.15.Iaddend.in which an air ring is used to cool the polymer afterextrusion thereof.
 12. The process of claim .[.9.]. .Iadd.15 .Iaddend.inwhich a smoothing roll is used to apply a smooth surface to the coating.13. The process of claim .[.9.]. .Iadd.15 .Iaddend.in which the polymerhas a density of at least 0.935 gm/cm³ and a melt index in the range0.2-30.
 14. The process of claim .[.9.]. .Iadd.15 .Iaddend.in which thepolymer has a density of at least 0.945 gm/cm³ and a melt index in therange 1-7.5 and that part of the metal block which contacts the sheetsubstrate in face-to-face contact is a rear extended die lip. .Iadd. 15.In a process for coating a substantially inflexible sheet substrate witha coating of thermoplastic polymer having a thickness of at least 0.25mm in which molten thermoplastic polymer is extruded between first andsecond die lips of an extrusion coating die and onto the face of saidsheet substrate, said extrusion die being formed, in part, of a metalblock having means to feed molten polymer therethrough to said die lips,the improvement comprising feeding a wood sheet substrate inface-to-face contact with said metal block over substantially the entirewidth of said sheet substrate, passing said sheet substrate from contactwith the metal block into spaced apart relationship with the first dielip, said sheet substrate thereby forming the second die lip of theextrusion coating die, the first die lip extending substantiallyparallel to and in spaced apart relationship from the sheet substratefor a distance of at least 10 cm, said sheet substrate having aplurality of openings selected from the group consisting of orifices orslits in the surface thereof, extruding molten thermoplastic polymerinto the space between the first die lip and the sheet substrate andthereby coating the sheet substrate with thermoplastic polymer..Iaddend.